For supporting placement of a base station, a radio apparatus and like in a radio communication system, used is a radio propagation characteristic estimating system (radio propagation simulator). Reception power or an expansion of a delay at an arbitrary reception point is evaluated by a radio propagation simulator to determine a position at which a transmission station is to be disposed, thereby ensuring a plane communicable area and achieving reduction in the number of base stations to be placed and the like.
Roughly speaking, radio propagation simulation is realized by a statistical method and a method using the decision theory.
Statistical method is a method of determining a parameter, with an estimation expression of a propagation loss provided to have a distance, a frequency or the like as an argument, by multivariate analysis or the like based on numerous data obtained by actual measurement of a propagation loss. While in general, radio wave propagation varies with place according to reflection on or transmission through an object, a median of variation with place is given by a method using the statistical theory. It is therefore impossible to precisely reflect an effect exerted on radio waves by a wall or an indoor object. As a result, the statistical method fails to estimate a radio wave propagation condition with high precision.
On the other hand, in the method using the decision theory, considering radio waves radiated from an antenna as a set of numbers of radio waves (rays) and assuming that each ray repeats reflection and transmission geometric-optically to propagate, rays reaching an observation point are composed to obtain a propagation loss or a delay. This method is called the ray tracing method. In the ray tracing method, since estimation is made taking into decision-theoretical consideration the effect caused by reflection, transmission and diffraction of an irradiated ray on an object within a region to be analyzed, radio wave propagation can be estimated with high precision. When there are many objects within a region to be analyzed, however, processing of searching a reflection point or a diffraction point is increased, resulting in increasing the amount of operation processing. As a result, radio wave propagation estimation will require much time.
In order to solve the problem in the method using the decision theory, Patent Literature 1, for example, discloses the method of speeding up propagation estimation by reducing the number of objects to be taken into consideration at the time of the ray searching processing. More specifically, recited is the ray searching processing executed by searching a road from a transmission point to a reception point and a path to a crossing in advance to limit a path of a ray and excluding an object apart from the searched path from targets to be considered. The method recited in Patent Literature 1 is effective when a height of a transmission point is smaller than a surrounding building such as a street micro cell.
On the other hand, as a usage of three-dimensional map drawing, Patent Literature 2, for example, discloses the method of reducing loads on drawing by visually combining buildings whose importance is low. More specifically, in the method, importance of an object is determined based on a volume (or surface area), complicity of a shape and attributes of the object to combine elements adjacent to each other with respect to an element whose importance is smaller than a predetermined threshold value. Attribute here is determined by a relative height with respect to an adjacent object, a distance to adjacent objects and whether it is at a crossing or not. In other words, an object whose volume is large, whose shape is complicated, whose height is larger than an adjacent object, which has no object in its surroundings or which faces the corner of the crossing is considered to have high importance and excluded from targets to be combined. Conversely, other objects than those described above are considered as low-importance objects and combined.    Patent Literature 1: Japanese Patent Laying-Open No. 09-33584.    Patent Literature 2: Japanese Patent Laying-Open No. 2006-284704.    Non Patent Literature 2: H. Hoppe, “Progressive Meshes”, Computer Graphics (Proc. ACM SIGGRAPH), pp. 99-108, 1996.
The method recited in Patent Literature 1, however, has a problem that in such a case of a macro cell often used in a cellular phone network where a height of a transmission point is larger than a surrounding building, an estimation error will be increased. The reason is that in the method of Patent Literature 1, effects of a building not in proximity to a road and a crossing which is excluded from a target to be considered will not be ignorable in propagation estimation in a macro cell.
The method recited in Patent Literature 2, which is basically a manner of reducing loads on three-dimensional map drawing, might have a building combined which will largely affect an estimation precision of radio propagation estimation. As a result, an estimation error will be naturally increased. More specifically, while when estimating a propagation condition of radio waves from a transmission point of a macro cell to a road, a building whose height is smaller than the transmission point is crucial, such a building is considered to have low importance and combined in the method recited in Patent Literature 2.
An object of the present invention is to provide a system, a method and a program for estimating a radio wave propagation condition at a high speed by reducing the number of constituent faces of an object which has a small effect on an estimation precision by using position data of a transmission point.